1. Field of the Invention
The invention relates generally to powered concrete finishing trowels and, more particularly, to hydraulically-driven riding trowels. The invention additionally relates to a hydrostatically driven riding trowel having a coolant system for cooling the trowel's hydraulic fluid. The invention additionally relates to a method of operating such a trowel.
2. Description of the Related Art
A variety of powered machines are available for smoothing or otherwise finishing “wet” or uncured concrete. These machines range from simple hand trowels, to walk-behind trowels, to self-propelled riding trowels. Regardless of the mode of operation of such trowels, the power trowels generally include one to three rotor assemblies that rotate relative to the concrete surface.
Riding concrete finishing trowels can finish large sections of concrete more rapidly and efficiently than manually pushed or guided hand-held or walk behind finishing trowels. Riding concrete finishing trowels typically include a frame having a cage that typically encloses two, and sometimes three or more, rotor assemblies. Each rotor assembly includes a driven shaft and a plurality of trowel blades mounted on and extending radially outwardly from the bottom end of the driven shaft. The driven shafts of the rotor assemblies are driven by one or more engines mounted on the frame and typically linked to the driven shafts by gearboxes or hydraulic pumps and motors of the respective rotor assemblies.
The weight of the finishing trowel, including the operator, is transmitted frictionally to the concrete surface by the rotating blades, thereby smoothing the concrete surface. The pitch of individual blades can be altered relative to the driven shafts via operation of a lever and/or linkage system during use of the machine. Such a construction allows the operator to adjust blade pitch during operation of the power trowel. As is commonly understood, blade pitch adjustment alters the pressure applied to the surface being finished by the machine. This blade pitch adjustment permits the finishing characteristics of the machine to be adjusted. For instance, in an ideal finishing operation, the operator first performs an initial “floating” operation in which the blades are operated at low speeds (on the order of about 30 rpm) but at high torque. Then, the concrete is allowed to cure for another 15 minutes to one-half hour, and the machine is operated at progressively increasing speeds and progressively increasing blade pitches up to the performance of a finishing or “burning” operation at the highest possible speed—preferably above about 150 rpm and up to about 200 rpm.
Power trowels traditionally were powered by a gearbox mechanically coupled to an internal combustion engine and were steered manually using a lever assembly coupled to the gearbox assemblies by linkage assemblies. More recently, larger trowels have been introduced that are potentially fatiguing to steer manually. These trowels typically are steered via hydraulically powered actuators responsive to operator manipulation of joysticks. Some of the hydraulically steered trowels are also powered hydraulically via a hydrostatic drive system powered by the machine's internal combustion engine(s). These trowels can be quite large. Some are capable of finishing swaths of 8 feet wide or even 10 feet wide or wider. They are powered by an engine having an output of over 50 hp, and sometimes in excess of 70 hp, and weigh more than 2,500 lbs.
The hydrostatic drive system of a riding trowel typically includes a cooling system for cooling the hydraulic fluid or oil being pumped through the drive system and the other hydraulically actuated components of the system. (The terms “hydraulic fluid”, “fluid”, and “oil” are used interchangeably throughout this disclosure). Some hydrostatically driven trowels employ a closed loop cooling system including a cooler in the closed loop between the drive motor(s) and drive pump(s) of the hydrostatic drive system. This is in contrast to the vast majority of hydrostatic drives that employ a cooler in an open loop branch of the hydraulic circuit. These riding trowels can be cooled via a closed loop cooler, despite the fact that the pressure at the outlet of the drive pump is far too high to be accommodated by known oil coolers suitable for use in equipment of this type, because fluid flow in the closed loop circuit is unidirectional. The low pressure or “charge” side of the circuit thus never experiences “load” pressure. That side of the circuit instead only reaches “charge” pressure, which is sufficiently low to be tolerated by some oil coolers. The resulting cooling system actively cools the highest flow as well as typically the hottest oil in the circuit.
However, the inventors have discovered that a closed loop cooling circuit alone may provide insufficient cooling of some larger hydrostatically driven riding trowels, particularly if the machine is operated for prolonged periods of time under extreme operating conditions such as under high ambient temperatures and/or on a surface having a high coefficient of sliding friction. Specifically, the inventors have discovered that the high duty cycles under heavy loads experienced by the hydrostatic drive system of a riding trowel can increase the temperature in the reservoir due to hot oil leakage from the pumps and motors. As there is no cooling in the open loop, the reservoir temperatures can rise to above 93° C. (200° F.). Hydraulic fluid viscosity drops with temperature, reducing the volumetric efficiencies of the system's charge pump and reducing the lubrication boundary layer for the parts to slide against each other in the tandem pump. The inventors have discovered that the fluid viscosity can drop so much in a power trowel having a single cooler that cavitation can occur. Accelerated piston shoe wear and even failure in the axial piston pump and other failures may occur. The inventors thus have discovered a need to prevent detrimental effects to a hydrostatically driven riding trowel that could result from overheating of the system's hydraulic fluid.
This need theoretically could be met by providing a larger oil cooler in the system's closed loop circuit. However, no known oil coolers on the market today have been found to be suitable to provide adequate cooling of the existing hydrostatic drive system during prolonged operation under extreme operating conditions in which the oil temperature in the reservoir is undesirably high.
Oil overheating and resultant viscosity drop also theoretically could be avoided by providing larger-capacity drive pumps and drive motors and otherwise “sizing up” components of the trowel's hydrostatic drive system. However, such a “sizing up” would add considerable cost to the overall system. It also would add weight, which is detrimental because adding weight to a riding trowel increases the time the concrete must cure before a finishing operation can commence. For example, a machine that currently weighs 2,700 lbs. that is modified to have larger-capacity pumps and hydraulic motors likely would weigh in excess of 3,300 lbs.
Therefore, the need remains to provide adequate cooling of hydraulic fluid in a hydrostatic drive system of a hydrostatically driven concrete finishing trowel.
The need additionally exists to provide adequate hydraulic fluid cooling in a hydrostatically driven riding power trowel that does not significantly increase the cost or weight of the machine.